Electronic circuits are often used to attenuate a selected band of frequencies while passing a larger band of frequencies. Filters employed for this purpose are commonly known as notch filters. A notch filter can best be defined as a filter having a single rejection band extending from a finite lower cutoff frequency (greater than zero) to a finite upper cutoff frequency. Frequencies within the rejection band are eliminated or attenuated, while frequencies outside the rejection band are retained.
Such notch filters are widely used in conjunction with cable television systems. Within a cable television system a broad range of carrier frequencies are provided, with each frequency corresponding to a service for which the subscriber pays. If a subscriber does not pay for selected services, it is well-known that placement of a notch filter in the coaxial line leading to the subscriber's facility will remove the frequencies corresponding to those services. If several frequencies need to be removed, the filters may be placed in series, or a single filter may be arranged to remove more than one frequency.
Cable television engineers have conceived a wide variety of notch filter networks or circuit designs in an attempt to satisfy the constantly changing demands for such devices. In some cable television systems, such filters are installed outside a customer's home or place of business, often on a pole or pad outside the home or business, to permit cable company personnel to have easy access to the devices. Since no source of energy is available, it is necessary that the filter is constructed using purely passive circuit components (i.e., resistors, capacitors, and inductors). At the same time, filters are exposed to the full range of variations in ambient temperatures. Variations in ambient temperature cause the values of the passive circuit components to change, which often results in de-tuning of the filter by causing shifting of the center frequency or otherwise adversely affecting the stability of the filter device. In order to ensure that the filter operates correctly, it is necessary that the shape of the frequency response produced by the filter remain relatively constant. Not only must such filter maintain tuning in the neighborhood of a desired center frequency; the filter must also retain the shape of the notch in terms of width and depth. Drifting of the center frequency could prevent reception of the desired frequencies of the signal by unnecessarily attenuating immediately adjacent frequency bands or channels. At the same time, the very portion of the spectrum which is sought to be attenuated might be allowed to pass. Thus, stability of these filters is highly desirable.
Since the advent of notch filters, cable television engineers have been faced with the conflicting goals of simplicity and performance. As a general rule, it has conventionally been accepted that stability is inversely proportional to the simplicity of the circuit. To achieve the desired performance characteristics it has been necessary to utilize complex notch filter-circuits having several poles which necessarily increased the number of components and overall size of the filter. For example, a four-pole device is shown in FIG. 4. Although the circuit shown in FIG. 4 has great stability for a passive notch filter design, it suffers from the undesirable tradeoff that a large number of components are required. FIG. 3 shows a conventional three-pole notch filter that provides good stability, but which also employs ten parts. Thus, it is clear that in each of the above designs a large number of components is necessary in order to stabilize the center frequency while maintaining the desired notch width and depth.
For quite some time, cable television engineers have unsuccessfully attempted to reduce the number of components by using a simpler two-pole notch filter. For example, the assignee of the present application has utilized the two-pole notch filter shown in FIG. 2. However, as shown in FIG. 5a, the “tuning window” or notch characteristic associated with this particular two-pole design was extremely narrow. Thus, the attenuation provided by the filter was insufficient to achieve the bandwidth necessary to ensure stable performance of the filter.
Known two-pole filters such as the one shown in FIG. 2 have insufficient stability to successfully function in cable or CATV applications. The prior art two-pole notch filter shown in FIG. 2 is very sensitive to environmental changes and susceptible of detuning over time due to its narrow range of optimum phase cancellation tuning and/or performance. A minor change in the frequency of either pole will cause significant bandwidth and attenuation performance changes of the filter and any system in which the filter is placed or serves. Even with significant component changes to increase the bandwidth, the prior art two-pole filter continues to be unstable and overly sensitive due to its narrow phase cancellation tuning/notch range.
In an effort to better improve the stability of the notch, the present assignee had previously cascaded two of these two-pole filters in series with each other (effectively providing the 4-pole filter shown in FIG. 4). It is well-known that the bandwidth and attenuation of the system will be doubled by cascading two such devices. The inventor found that adequate attenuation (−50 dB) could be achieved by cascading two two-pole devices. Although this practice greatly improved the stability of the filter, while providing the desired attenuation and bandwidth, this design also require the use of a relatively large number of components. This design also required the use of shielding between each two-pole section to prevent interfilter detuning. In turn, this increased the overall size and production cost of the filter. At the same time, cascading two of the two-pole filters resulted in a particularly wide notch characteristic in the vicinity of the center frequency. This is undesirable since it causes attenuation or even complete elimination of immediately adjacent residual side bands necessary for the reception of adjacent channels.
In view of the above-described problems, it is clear that the prior art notch filters often exhibit inadequate stability and/or inadequate notch sharpness, as in prior two-pole devices, or are of undesirably large size and high production cost, such as the present four-pole devices. Thus, a great need exists for a notch filter that will improve the stability and notch sharpness problems associated with known two-pole devices, while simultaneously reducing the size and production cost associated with known four-pole devices.